In VPSA processing, a feed gas mixture containing a more readily adsorbable component and a less readily adsorbable component is passed to an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at a higher adsorption pressure. The bed is thereafter depressurized to a lower desorption pressure for desorption of the more readily adsorbable component and its removal from the bed, prior to repressurization and the introduction of additional quantities of the feed gas mixture to the bed, as cyclic adsorption desorption operations are continued in the bed.
Such VPSA processing is commonly carried out in multi-bed systems, with each bed employing the same VPSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in the other beds of the adsorption system. In VPSA systems for the separation of air, adsorbents have been employed that selectively adsorb nitrogen as the more readily adsorbable component, with oxygen being recovered as the less readily adsorbable component. Zeolitic molecular sieves, which operate on an equilibrium basis with a front of the selectively adsorbed nitrogen forming and advancing in the bed from the feed end to the product end thereof, are of this type and can be used in VPSA processing cycles for the production of either oxygen or nitrogen as the desired product. In the latter case, an oxygen enriched air stream is also recovered.
A low-cost method is needed to make low-purity oxygen at a steady rate from air. An example of useful low-purity oxygen is a gas stream containing 40% oxygen, which can be used as the oxidizer for a combustion process. Such a gas stream contains almost twice as much oxygen as does air and the ratio of nitrogen to oxygen is less than half as great as in air. Since there is less nitrogen for a given amount of oxygen, there is less combustion energy lost in heating the nitrogen. Also there is less flue gas to dispose of; the burner does not need to be as large; and a higher combustion temperature may be reached.
Typically, low-purity enriched oxygen is produced in two steps, by producing high-purity oxygen and then blending it with air to produce a stream of low-purity oxygen. An improved method for producing moderate amounts of enriched, low-purity oxygen is disclosed in U. S. Pat. Nos. 4,867,766 and Re 34,434 to Campbell et al. Therein, higher-purity oxygen (typically about 90 to 95 mol %) is produced by a pressure swing adsorption (PSA) system and the higher-purity oxygen is blended with air to produce a low-purity oxygen stream. Other, earlier methods for producing low-purity oxygen are mentioned therein as prior art which involve the blending of high-purity oxygen (typically at least 99.5 mol %) with air to produce the low-purity oxygen stream.
U.S. Pat. No. 5,382,280 to Choe et al. describes the use of an equilibrium-based, oxygen-selective adsorbent to remove oxygen from a gas stream in the second stage of a process for producing nitrogen from air. The adsorbent is described as having a Langmuir Type 1 shape, an infinite selectivity and accepts only about 1% to 5% oxygen in the feed. Choe et al. further teach a purge step that removes "any oxygen which may remain" in the unit. Such a thorough purge produces a rapid decline in the oxygen content of the effluent gas toward the end of the purge step.
Ruthven, et al. describe a pressure-swing process to concentrate hydrogen from a mixture with helium. See "Concentration of a trace Component by Pressure-Swing Adsorption", Ruthven, D. M. et al., Chemical Engineering Science (OXFORD) Vol. 49 No. 1, January 1994, pp. 51-60. The adsorbent used is zeolite 5A. The pressure-swing cycle runs at 77 K. The hydrogen isotherm is "well represented by the Langmuir expression". The adsorbent is highly selective for hydrogen over helium. Nevertheless, during the constant-pressure purge step, the hydrogen concentration in the effluent falls rapidly.
Neither Ruthven et al. nor Choe et al. illustrate a successful attempt to run a pressure-swing cycle with a desorption purge step that produces a steady stream of gas with a nearly-constant concentration of the more-strongly-adsorbed component.
Accordingly, it is an object of this invention to provide an improved method for the production of a preferred gas-enriched product from a mixture of the preferred gas and a less preferred gas.
It is another object of this invention to produce an oxygen enriched product without a need for first producing high-purity oxygen.
It is a further object of this invention to produce an oxygen enriched product by a process which avoids a blending step that generates entropy of mixing and thereby wastes energy.
It is yet another object of this invention to produce an oxygen enriched product with a process which produces a steady product stream directly from air in one adsorption cycle and can use compressors operating with low compression ratios.